docs: def helical_turbulent_fd_Prasad(Re, Di, Dc,roughness=0):
      r'''Calculates Darcy friction factor for a fluid flowing inside a curved 
      pipe such as a helical coil under turbulent conditions, using the method of 
      Prasad [1]_, also shown in [2]_.
          
      .. math::
          f_{curv} = f_{\text{str,turb}}\left[1 + 0.18\left[Re\left(\frac{D_i}
          {D_c}\right)^2\right]^{0.25}\right]
          
      Parameters
      ----------
      Re : float
          Reynolds number with `D=Di`, [-]
      Di : float
          Inner diameter of the coil, [m]
      Dc : float
          Diameter of the helix/coil measured from the center of the tube on one
          side to the center of the tube on the other side, [m]
      roughness : float, optional
          Roughness of pipe wall [m]        
  
      Returns
      -------
      fd : float
          Darcy friction factor for a curved pipe [-]
  
      Notes
      -----    
      No range of validity was specified, but the experiments used were with 
      coil/tube diameter ratios of 17.24 and 34.9, hot water in the tube, and
      :math:`1780 < Re < 59500`. At very low curvatures, converges on the
      straight pipe result.
  
      Examples
      --------
      >>> helical_turbulent_fd_Prasad(1E4, 0.01, .2)
      0.043313098093994626
  
      References
      ----------
      .. [1] Prasad, B. V. S. S. S., D. H. Das, and A. K. Prabhakar. "Pressure 
         Drop, Heat Transfer and Performance of a Helically Coiled Tubular 
         Exchanger." Heat Recovery Systems and CHP 9, no. 3 (January 1, 1989): 
         249-56. doi:10.1016/0890-4332(89)90008-2.
      .. [2] El-Genk, Mohamed S., and Timothy M. Schriener. "A Review and 
         Correlations for Convection Heat Transfer and Pressure Losses in 
         Toroidal and Helically Coiled Tubes." Heat Transfer Engineering 0, no. 0
         (June 7, 2016): 1-28. doi:10.1080/01457632.2016.1194693.
      '''
      fd = friction_factor(Re=Re, eD=roughness/Di)
      return fd*(1. + 0.18*(Re*(Di/Dc)**2)**0.25)